While the adult mammal is known to have control systems which determine what it eats, when it eats and how much it eats, our knowledge of these mechanisms in the infant is scanty. Studies in rats have shown that the adult ability to balance intake with caloric needs is not functional during suckling but matures at weaning. In view of this we propose that maternal factors regulate milk intake. We will investigate whether: a) there is a satiety factor in milk, b) the dam limits milk intake by terminating suckling, and c) milk availability determines intake. Our second aim is to study the ontogeny of feeding controls in two other species: the guinea-pig because of its early maturation, and the rabbit because it suckles only once a day. Our third aim is to use rats to determine whether development of the mechanisms for regulating caloric intake is adversely affected by introduction, during the suckling/weanling period, of either: a) a sweet diet, b) a novel diet, c) excess milk, or d) stress. In these first three components of the program, the methods will involve the comparison of feeding behavior of fed and fasted animals. The parameters to be measured are: i) video-analysis, ii) continuous event recording, and iii) analysis of body composition. Chemical studies of milk will assess the presence of a satiety factor. Temperature recording from dams will determine the role of maternal hyperthermia in the termination of suckling bouts. Our fourth aim is to elucidate the physiological processes underlying weaning in the rat. We will determine if: a) it is cued by a maternal pheromone, b) it represents the maturation of a specific appetite for carbohydrate in the pups, and c) increasing pup size aggravates the maternal hyperthermia of suckling. Weaning will be assessed by measuring the percent chow in stomach contents. Olfactory discrimination apparatus will be employed in pheromone studies; selection of special liquid diets will assess carbohydrate appetite; thermal manipulations and recordings will be used for the third hypothesis. Our final aim is to determine when the young rat acquires the ability to regulate its intake of protein and carbohydrate. These investigations will utilize classical techniques of diet pair selection and "cafeteria" studies.